1. Field of the Invention
This invention relates to fire detectors of the kind having transducer means, and capable of giving a reaction at ambient temperatures to changes in an atmospheric condition characteristic of the presence of combustion, with the transducer means being arranged to give an electrical output signal representing its response to the said changes. Such a fire detector will be referred to as a fire detector of the kind specified.
2. Discussion of the Prior Art
Commercially available fire detectors generally rely on purely physical principles, by detecting smoke particles or a significant increase in atmospheric temperature, or alternatively, to a lesser extent, by sensing heat or infra-red radiation generated by a fire. Common difficulties with such devices arise from spurious effects, e.g. presence of dust particles or heat emanating from sources other than a fire etc., all of which can lead to false alarms.
It is also known that a fire can be detected in some (though not all) cases by detection of gases produced by the combustion involved, and that this can often afford a means of obtaining very early warning. Studies to demonstrate this are described in the papers of Fardell et al., Fire and Materials, 10 (1986) p. 21-8, Hurst et al., Fire and Materials, 9 (1985) p. 1-8, and Harkoma et al., Combust. Sci. and Tech., 62 (1988), p. 21-9. It has been demonstrated specifically that carbon monoxide sensors can be used to detect fires, as described in Harwood et al., Fire Safety Journal 17 (1991) p. 431-443. In this last-mentioned paper, the sensors are of the impregnated semiconductor type which react to a change in the concentration of carbon monoxide in the atmosphere by a change in their electrical resistance.
Again, it is known that the incidence of false alarms can be reduced, and the security of detecting a fire improved, by using a combination of sensors of different types either in an array, or in single sensors of different type disposed around a building.
A fire variously produces gases (particularly carbon monoxide, and also, commonly, hydrogen), and smoke. Water vapor is often produced by combustion of hydrogenous matter such as hydrocarbons. In addition, of course, there will usually be an increase in atmospheric temperature. The extent to which any one of these factors is present to any significant, or easily-detectable, extent will vary according to the circumstances. This is especially so in the very early stages of a fire when, of course, it is most desirable that the fire be detected: the factor that is predominant at that time should be detected regardless of which factor it happens to be.
Sensors of the impregnated semiconductor resistor type for sensing changes in carbon monoxide (CO) concentration are essentially chemical in function, albeit producing a physical effect in the form of an electrical signal representing the resistance of the sensor. It is known to be desirable to be able to diagnose a fire chemically, using sensors with suitable characteristics, with a view to avoiding the disadvantages of fire detectors that rely on purely physical principles as mentioned above.
The characteristics which are important for fire sensors employing chemical principles include robustness against poisons in the air, sensitivity to gases given off in typical fires, very low power consumption, small size and low cost. While some of these characteristics are present in the general type of sensor described in the above-mentioned paper of Harwood et al., the known art of fire detection using such a sensor still envisages a sensor dedicated to the detection of a single effect characteristic of combustion, e.g. CO concentration. To detect any other effect (such as temperature), it has been thought that separate sensing means would be essential.
Sensors (transducer elements, semiconductor resistors) of porous tin dioxide (SnO.sub.2), impregnated with at least one noble metal deposited on the surface of its pores and capable of giving a reaction at ambient temperatures to changes in the concentrations of particular gases in the atmosphere, and methods of making them, are described in the British patent specifications GB 2 249 179A and GB 2 248 306A.
In the first of those documents, a tin dioxide sensor element is impregnated with platinum and an additive which increases its selective sensitivity to the presence of atmospheric hydrogen, by increasing its density. In GB 2 248 306A, a tin dioxide resistor for use as a transducer or sensor element is impregnated with at least one phase of a metal such that the electrical resistance of the sensor is a function of the concentration of a given gas in the atmosphere in such a way that its sensitivity to that gas in trace quantities is reduced, but its sensitivity to the same gas is not impaired at the higher concentrations at which the sensor is most useful.